1. Field of the Invention
The present invention relates to power control circuits, and more specifically to power control circuits for low voltage lighting systems. Even more specifically, the present invention relates to power control circuits for low voltage lighting systems having variable loads.
2. Discussion of the Related Art
Low voltage lighting systems, such as found in outdoor lighting systems, typically include electrical loads that use a lower voltage than the standard house current circuit that provides 120 volts AC. Such low voltage lights commonly require a 12 volt AC input.
A transformer is used to convert power at 120 volts AC to power at 12 volts AC that is used for the input at the electrical loads, e.g. the lamps. Typically, in outdoor lighting applications, the lamps are coupled to the transformer via parallel wiring and are located at varying distances from the transformer. The wiring supporting a string of lamps is referred to as a wiring run or a lighting run. Disadvantageously, a lamp load that is electrically remote from the transformer will burn more dimly than an identical lamp load electrically close to the transformer. This results from the resistive loss and consequent voltage drop in the wiring from the transformer to the lamp load. The longer the distance from the transformer, the larger the voltage drop across the wires. Thus, the effective voltage at the lamps may be less than 12 volts AC.
Furthermore, low voltage lighting systems are designed to be flexible in the number and positioning of lamp loads with respect to distance along power distribution wires. However, by changing the number of lamp loads present in the system, the total voltage drop is varied, resulting in an overall brightness change in the remaining lamp loads. For example, an increase in lamp loads with the same power signal, results in a lower voltage at the lamp loads such that each lamp will appear to dim. Additionally, if one lamp load blows, the voltage in the remaining lamps will increase, leading to the remaining lamps running brighter. The increased voltage in the remaining lamps may contribute to their premature failure.
One solution is to distribute power at a high voltage, but locally reduce the voltage for each lamp load. This results in running a high voltage signal from the standard household current circuit to the lamp loads, leading to increased risk of electrical shock in the event of a person accidentally coming into contact with a the wiring carrying the high voltage power signal. Disadvantageously, such a solution also requires a separate transformer at each lamp load to convert the high voltage power signal to a voltage level useable by the respective lamp load.
Another solution is to control the voltage applied to the primary windings of the transformer. Typically, sensing wires are used to sense the voltage applied to the lamp loads and provide feedback to a controller coupled to the primary windings of the transformer. This controller controls the voltage applied to the primary windings which controls the output voltage of the secondary circuit such that the voltage at the lamp loads is at the desired level. Thus, the output voltage is greater than 12 volts AC, but is about 12 volts AC after the voltage drop across the wires. This approach uses triacs at the primary windings and disadvantageously requires additional sensing wires to run from the remotely located electrical loads back to the transformer. Furthermore, since each lighting run will have a respective voltage drop associated with it, each lighting run requires a separate transformer to convert the 120 volt AC power signal to a voltage level that once the voltage drop of the specific lighting run is accounted for, will be about 12 volts AC. Each of the separate transformers is controlled by its own triac in the primary and has its own sensing wires, which significantly add to the cost of the system.
The present invention advantageously addresses the needs above as well as other needs by providing a constant voltage controller coupled to and located remotely from the secondary of a power transformer for maintaining the voltage at a plurality of electrical loads at a constant level regardless of the voltage drop associated with powering the plurality of electrical loads.
In one embodiment, the invention can be characterized as a power control system including a power transformer having a primary portion and a secondary portion and a wiring run having a length and coupled at one end to the secondary portion. Also included is a constant voltage controller coupled to another end of the wiring run. The constant voltage controller receives an input power signal from the power transformer and outputs an output power signal at a specified voltage level to a plurality of low voltage electrical loads. The input power signal is at a higher voltage level than the specified voltage level and the output power signal is maintained at the specified voltage level regardless of a change in a voltage drop of the power control system.
In another embodiment, the invention can be characterized as a constant voltage controller for maintaining a power signal at a constant voltage at a plurality of electrical loads including a switch receiving an input AC power signal from a secondary portion of a power transformer and outputting an output AC power signal to the plurality of electrical loads. Also included are a controller coupled to the switch and a first logic rectifier circuit coupled to the controller for inputting the output AC power signal and a return AC power signal and outputting a voltage feedback signal to the controller. A second logic rectifier circuit is coupled to the controller and inputs the output AC power signal and outputs a current feedback signal to the controller. And, a reset controller including a third logic rectifier circuit is coupled to the controller for inputting the input AC power signal and outputting a reset signal to the controller. The controller maintains a voltage level of the output AC power signal at a specified level by controlling the operation of the switch based upon the reset signal, the current feedback signal and the voltage feedback signal.
In a further embodiment, the invention can be characterized as a method of controlling the voltage of a power signal for a plurality of electrical loads comprising the steps of: receiving an AC power signal from a secondary portion of a power transformer, wherein the AC power signal is at a voltage greater than a specified level corresponding to the plurality of electrical loads; adjusting the voltage of the AC power signal to a voltage level such that the voltage supplied to the plurality of electrical loads is at the specified level; and maintaining the voltage of the AC power signal at the specified level regardless of a change in a voltage drop experienced by the AC power signal.
In yet another embodiment, the invention can be characterized as a voltage feedback for a power control system for outdoor lighting systems including a switch inputting an input AC power signal and for providing an AC output power signal to a plurality of lamp loads and a controller coupled to the switch and controlling the operation of the switch. Also included is a logic rectifier circuit coupled to the controller wherein the logic rectifier circuit inputs the AC output power signal and a return AC power signal from the plurality of lamp loads. The logic rectifier circuit provides a rectified voltage feedback signal for the controller which is used to adjust the operation of the switch.
In yet another further embodiment, the invention can be characterized as a current feedback for a power control system for outdoor lighting systems including a switch inputting an input AC power signal and for providing an AC output power signal to a plurality of lamp loads and a controller coupled to the switch for controlling the operation of the switch. Also included is a logic rectifier circuit coupled to the controller wherein the logic rectifier circuit inputs the AC output power signal across a current sense resistor. The logic rectifier circuit provides a rectified current feedback signal for the controller which is used to adjust the operation of the switch.
In a subsequent embodiment, the invention can be characterized as a power controller for outdoor lighting systems including a switch inputting an input AC power signal and for providing an AC output power signal to a plurality of lamp loads and a controller coupled to the switch for controlling the operation of the switch. Also included is a logic rectifier circuit coupled to the controller wherein the logic rectifier circuit inputs the AC input power signal and outputs a reset signal for the controller. The reset signal comprises a pulse signal sent to the controller at the beginning of each half cycle of the input AC power signal.